As a light source dimming method in, for example, a liquid crystal display device, a method is described in Patent Document 1 in which an average luminance value is calculated from a video signal that is acquired during a fixed time interval and the luminance of the backlight is then controlled on the basis of the average luminance value. In this control method, the luminance of the backlight is decreased when the average luminance value surpasses a threshold value, and the luminance of the backlight is increased when the average luminance value falls below a threshold value. In this way, the brightness of the screen can be adjusted to the optimum brightness according to the luminance level of the video.
In the above-described control method, however, because the luminance of the backlight is controlled only by the average luminance value of the video signal regardless of the characteristics of the image, the optimum brightness is not achieved in some cases due to image characteristics.
For example, a first image in which the central region is white and the peripheral regions are black and a second image in which the entire image is an intermediate color such as grey may have the same average luminance even though the image characteristics thereof are different from each other. Normally, in order to display these first and second images at optimal brightness, the luminance of the backlight must be appropriately controlled for each of the first and second images. In the control method described above, however, the luminance of the backlight for these first and second images will be the same, and display of the first and second images at the optimum brightness will therefore encounter problems.
In response, a liquid crystal display device has been proposed in Patent Document 2 in which the luminance of the backlight is adjusted by using, in addition to the average luminance value, histograms that indicate the characteristics of image.
In the liquid crystal display device described in Patent Document 2, a three-level histogram is created on the basis of an input video signal. Here, the three-level histogram is of a form in which the levels of brightness are represented by the three levels of low luminance, intermediate luminance, and high luminance, and each picture element value of image data is graphed by the different levels.
The characteristics of an image are determined on the basis of a three-level histogram, and the light quantity of the backlight is adjusted on the basis of the characteristics quantity of the image that is indicated by the frequency of predetermined brightness levels (for example, the frequency of the high-luminance side) of the three-level histogram. In this way, control of the light quantity according to the characteristics of each image can be carried out for the above-described first and second images whose characteristics are different from each other.